This invention relates to a raster scanner, and more particularly, to a raster scanner optical system which corrects a focus error caused by manufacturing tolerances of the optical elements of the raster scanner.
Raster scanner optical systems contained in the prior art have a light source, such as a laser, which emits a coherent light beam. The beam is collimated in both the fast-scan or tangential plane and in the cross-scan or sagittal plane by multiple optical elements. The collimated beam in the cross-scan plane is focussed at a point near a facet of a rotating polygon mirror by a cylindrical optical element while in the fast-scan plane the light beam remains collimated when the beam strikes the facet of the rotating polygon mirror.
The rotating polygon mirror causes the reflected beam to revolve about an axis near the reflection point of the rotating polygon mirror. This reflected beam can be utilized to scan a document at the input end of an imaging system as a raster input scanner or can be used to impinge upon a photosensitive medium, such as a xerographic drum (photoreceptor), in the output mode as a raster output scanner.
Typically, an optical element has manufacturing tolerances such as surface radii, element thickness or refractive index which can cause a slight deviation in the divergence of a light beam. In a raster scanner, which has a plurality of optical elements, the error caused by each optical element is cumulative. Consequently, manufacturing tolerances of the optical elements can substantially shift the focal point of the light beam from a point on the photoreceptor plane to a point outside of the photoreceptor plane which degrades the quality of the printed document.
Depending on the parameters of the raster scanner, the depth of focus, which defines the acceptable range of the focus error, varies. The low resolution raster scanners (i.e. 400 SPI) have a larger depth of focus compared to high resolution raster scanners (i.e. 600 SPI). Therefore, the low resolution raster scanners are more tolerant to the focus errors. On the contrary, the high resolution raster scanners are frequently impacted by the manufacturing tolerances of the optical elements. Usually, in a high resolution raster scanner with a small allowable depth of focus, the amount of the focus error is larger than the depth of focus.
To solve this problem, an option is to use optical elements with tight tolerances. However, this recourse is cost prohibitive and impractical.
Furthermore, the acceptable focus error in the fast-scan plane may be different than the focus error in the cross-scan plane. Since different optical elements are utilized to differentiate the light beam in the fast-scan plane from the light beam in the cross-scan plane, the conjugates in the fast-scan plane are different than the conjugates in the cross-scan plane. Therefore, the focus error in the fast-scan plane has to be corrected in a different manner than the focus error in the cross-scan plane.
Also, if the spot is elliptically shaped, the size of the spot in the fast-scan plane is smaller than the size of the spot in the cross-scan plane. Consequently, the allowable depth of focus in the fast-scan plane is smaller than the allowable depth of focus in the cross-scan plane which makes it more challenging to correct the focus error in the fast-scan plane than in the cross-scan plane.
Ordinarily, a pre-polygon cylindrical lens which only affects the light beam in the cross-scan plane is used to correct the focus error in the cross-scan plane. However, the focus error in the fast-scan plane remains unaffected. To "correct the focus error" as used herein shall mean moving the focus into the range defined as"allowable depth of focus" which is the acceptable range of the focus error.
It is an object of this invention to provide optical means to correct the focus error, caused by the manufacturing tolerances of the optical elements of the raster scanner, in both the fast-scan and cross-scan planes.